This is not a proper guide about how to obtain this achievement.
To get this you basically need to do all the Mathias missions but, due the glitchy / buggy design nature of the game you perhaps will be facing some issues trying to activate it.
It happened to me and I have to put some effort on it, so spoilers below, just check it in the case you are following Raban Maur missions and you are stuck trying to trigger the final one with Matthias.
*** Spoiler - click to reveal ***
As you may notice, the original Matthias from the game was splitted into three different characters.
When the last mission is activated you will see the dialog icon over the map directions and when reaching "them", you will se perhaps some of them alone in a spot, and a frame later it vanishing.
This is an awful bug from the game. Basically you see one of them and when approaching to activate the dialoge options with it disappears
You will be able to press the button before the vanish if you approach slowly but, in the middle of the first line of dialogue it will dissapear anyway.
The solution I've found was, to do this to the "alone Matthiases", 2 times from the entrance, and then there is a center spot when sometimes they are together and, it will be triggered another dialoge option on the rightmost character.
After two unsuccesful attempts, when these are together and looking at the right one will activate the mission in the right way
Sorry for this but dealing with this kind of bugs at version 1.7 of the game (august 2020) it is beyond me why it is not fixed